log_link: fix the log_link test failure on adsp boards

The log_link tests are meant to work with 2 mock log backends.
But Intel ADSP platform has an extra default backend named
"log_backend_adsp" when CONFIG_LOG_BACKEND_ADSP=y, which will
mess things up.

Fix it by disabling the default backend.

Signed-off-by: Ming Shao <ming.shao@intel.com>